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The coefficient of safety is an important index for the evaluation of slope stability. The safety coefficients calculated based on different instability criteria are significantly different, and the variability of the results will directly affect the evaluation of the stability state of slopes. In this paper, taking the step-type excavated slope of the Wanhuashan Trench Construction Project as the object, FLAC3D finite difference software was used to calculate the safety coefficient of loess slopes based on three different types of instability criteria, namely the calculation non-convergence criterion, the plastic penetration criterion and the displacement abruptness criterion, and to compare and analyze the differences, advantages and disadvantages of the evaluation results of the three criteria. The results of the study shows that there is a certain difference in the magnitude of safety coefficients obtained by using different instability criteria, with the order of magnitude being displacement abrupt change criterion > computational non-convergence criterion > plastic penetration criterion. The computational non-convergence criterion is easy to use and fast to solve, but does not show the change in the plastic zone of the slope during the discounting process. The plastic penetration criterion is not suitable as a criterion for slope instability on its own, but needs to be combined with the unbalanced force rate to be assessed, but it can show the area of shear damage and stretching of the slope. The sudden change of displacement criterion is tedious to solve, but the solution process truly reflects the critical damage state due to the change of slope displacement. For practical conditions such as loess slopes, it is recommended to use the fast computational non-convergence criterion to evaluate the slope stability in combination with the plastic zone penetration and the slope displacement change. [ABSTRACT FROM AUTHOR]